This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Currently, wireless communication networks or systems operating at high frequencies from 10-300 GHz are emerging as a promising technology to meet exploding bandwidth requirements by enabling multi-Gb/s speeds. For example, the 5th Generation (5G) network is likely to be a combination of the evolved 3rd Generation (3G) technologies, the 4th Generation (4G) technologies and emerging or substantially new components such as Ultra-Density Network (UDN).
Due to operating at high frequencies, network nodes (including terminal devices and network devices) will be deployed with high density in the 5G network. Considering the fact that the power energy cost takes a large ratio of Operating Expense (OPEX) for communication operators, the power efficiency becomes a focus in the initial design of the 5G network and the even future communication networks operating at high frequencies. Currently, in order to reduce energy consumption in such networks and to fully enable utilizing high gain beam forming or other multi-antenna techniques, a concept has been defined by separating the control/broadcast layer from the data plane. The broadcast layer consist of a broadcasted access information table (AIT) and a broadcasted system signature (SS). The system signature is used as synchronization signal for access node discovery and the AIT contains the mandatory information for radio connection setup.
FIG. 1 shows timings of AIT and SS. The SS may be used to map information from the broadcasted AIT. The broadcasted signals should be able to send in a single frequency network (SFN) structure. Broadcasted information may contain parameter settings related to how to access a network device (random access) and be reached by the network device (paging). To reduce network energy consumption the broadcasted signals are expected to be infrequent compared to today's reference signals in cellular systems. The access information tables are typically transmitted with long periodicity. For example, AIT may be transmitted with a very long period, such as every 10.24 seconds, and SS may be broadcasted every 100 milliseconds to indicate the entry in the table and also provide the synchronization.
The SS may be assigned with a unique index (referred to as System Signature Index (SSI)) by the higher layer, and terminal devices may use SSI to determine the entry in the AIT to find parameters for the following initial random access (also referred to as access related information). The parameters may include but not limited to, the basic system information, the random access preamble settings, the time-frequency allocation for the preamble transmissions and scanning, etc.
Since the system information for random access, i.e., AIT, is very sparse to enhance the power efficiency as an important feature in the 5G network, there will result in a large access delay for terminal devices, which are newly powered up, have no latest AIT and/or newly enter a different communication coverage. Because the terminal devices have to wait for possible maximum 10.24 s, i.e., the AIT transmission period before doing random access, the delay would be not acceptable, even for the delay tolerate services such as short messages, web browsing, etc. Therefore, it is much important to ensure the random access delay within an acceptable range.
FIG. 2 illustrates an example of the initial access delay of a newly powered up terminal device.
In the normal AIT, the entry would include a flexible set of random access parameters, including the preamble candidates, power, time (e.g., subframe), frequency (e.g., band) and spatial (e.g., beam) resources for all possible random access configurations, whose combination is indicated by the SSI. The combination needs to be flexible enough for all terminals to access the network device according to the scenario and requirements. Based on the successful detection on the SSI information, the terminal device knows in which time slot, band and transmit beam direction to send which sequence with the proper power. Correspondingly, the network device, e.g., eNodeB (eNB) (or equivalently speaking, Access Point (AP)), would scan received signals to detect whether there is any access request or not in the time slot, band and the beam receiving direction. To have as the better link performance as possible, the resource reserved for the random access can be optimized in the network device, such as optimized beam direction for the high beamforming gain with narrow beam. However, if there are many candidate beams, such as in a network device with massive Multiple Input Multiple Output (MIMO), the beams reserved for the random access have to be swept, since locations of terminal devices for the access request are not available. Such a flexibility results in the fact that the terminal devices might not know how to access the system before the AIT is determined.